Switching mode power supply with a multi-mode controller

Abstract

A switching mode power supply with a multi-mode controller is provided. The switching mode power supply may include a transformer having a primary winding and a secondary winding to supply power to a load. A feedback circuit may be included to generate a feedback signal that varies in relation to the load on the secondary winding. The multi-mode controller may include a switching circuit, a frequency control circuit and a current limiting circuit. The switching circuit may be coupled to the primary winding to control current flow through the primary winding. The frequency control circuit may control a switching frequency of the switching circuit based on the feedback signal. The current limiting circuit may limit current flow through the primary winding by causing the switching circuit to suspend current flow through the primary winding when the current reaches a peak current limit that is set based on the feedback signal.

It is claimed: 
     
       1. A switching mode power supply, comprising:
 a transformer having a primary winding and a secondary winding to supply power to a load; 
 a feedback circuit that generates a feedback signal that varies in relation to the load on the secondary winding; 
 a switching circuit coupled to the primary winding to control current flow through the primary winding; 
 a frequency control circuit that controls a switching frequency of the switching circuit based on the feedback signal; 
 a current limiting circuit that limits current flow through the primary winding by causing the switching circuit to suspend current flow through the primary winding when the current reaches a peak current limit that is set based on the feedback signal; 
 a burst mode control circuit that bypasses the frequency control circuit and the current limiting circuit to control operation of the switching circuit when the load on the secondary winding falls below a minimum load threshold; and 
 wherein the switching mode power supply operates in three modes: 
 a first mode during which the switching frequency varies as a function of the feedback signal and the peak current limit is constant; 
 a second mode during which both the switching frequency and the peak current limit vary as a function of the feedback signal; and 
 a burst mode during which operation of the frequency control circuit and the current limiting circuit is suspended. 
 
     
     
       2. The switching mode power supply of  claim 1 , wherein the switching mode power supply transitions from the first mode to the second mode to the burst mode as the load on the secondary winding decreases. 
     
     
       3. The switching mode power supply of  claim 1 , wherein the switching mode power supply further operates in an additional third mode during which the peak current limit varies as a function of the feedback signal and the switching frequency is constant. 
     
     
       4. The switching mode power supply of  claim 3 , wherein the switching mode power supply transitions from the first mode to the second mode to the third mode to the burst mode as the load on the secondary winding decreases. 
     
     
       5. The switching mode power supply of  claim 3 , wherein the switching mode power supply transitions from the third mode to the second mode to the first mode to the burst mode as the load on the secondary winding decreases. 
     
     
       6. The switching mode power supply of  claim 1 , wherein the frequency control circuit includes a voltage-controlled oscillator circuit that controls the switching frequency of the switching circuit as a function of a threshold voltage. 
     
     
       7. The switching mode power supply of  claim 6 , wherein the threshold voltage is determined by the feedback signal during at least one operational mode of the switching mode power supply, and wherein the threshold voltage held constant during at least one other operational mode of the switching mode power supply. 
     
     
       8. The switching mode power supply of  claim 7 , wherein the threshold voltage is held constant by a Zener diode circuit. 
     
     
       9. The switching mode power supply of  claim 7 , wherein the threshold voltage is held constant by a voltage source. 
     
     
       10. The switching mode power supply of  claim 1 , wherein the current limiting circuit includes a comparison circuit that controls the peak current limit as a function of a threshold voltage. 
     
     
       11. The switching mode power supply of  claim 10 , wherein the threshold voltage is determined based on the feedback signal during at least one operational mode of the switching mode power supply, and wherein the threshold voltage held constant during at least one other operational mode of the switching mode power supply. 
     
     
       12. The switching mode power supply of  claim 11 , wherein the threshold voltage is held constant by a voltage source. 
     
     
       13. The switching mode power supply of  claim 1 , wherein the burst mode control circuit includes a comparison circuit that suspends current flow through the secondary winding based on the feedback signal. 
     
     
       14. A switching mode power supply, comprising:
 a transformer having a primary winding and a secondary winding to supply power to a load; 
 a feedback circuit that generates a feedback signal that varies in relation to the load on the secondary winding; 
 a switching circuit coupled to the primary winding to control current flow through the primary winding; 
 a frequency control circuit that controls a switching frequency of the switching circuit based on the feedback signal; 
 a current limiting circuit that limits current flow through the primary winding by causing the switching circuit to suspend current flow through the primary winding when the current reaches a peak current limit that is set based on the feedback signal; 
 a burst mode control circuit that bypasses the frequency control circuit and the current limiting circuit to control operation of the switching circuit when the load on the secondary winding falls below a minimum load threshold; and 
 wherein the switching mode power supply operates in three modes: 
 a first mode during which the peak current limit varies as a function of the feedback signal and the switching frequency is constant; 
 a second mode during which the switching frequency varies as a function of the feedback signal and the peak current limit is constant; and 
 a burst mode during which operation of the frequency control circuit and the current limiting circuit is suspended. 
 
     
     
       15. The switching mode power supply of  claim 14 , wherein the switching mode power supply transitions from the first mode to the second mode to the burst mode as the load on the secondary winding decreases. 
     
     
       16. A method for controlling a switching mode power supply, comprising:
 regulating an output voltage of the switching mode power supply by switching a transformer on and off at a switching frequency; 
 generating a feedback signal that varies in relation to a load coupled to the output voltage; 
 controlling the switching frequency based on the feedback signal; and 
 limiting a peak current through a primary winding of the transformer based on the feedback signal; and 
 wherein the switching mode power supply operates in three modes: 
 a first mode during which the switching frequency varies as a function of the feedback signal and the peak current limit is constant; 
 a second mode during which both the switching frequency and the peak current limit vary as a function of the feedback signal; and 
 a burst mode during which the switching frequency and the peak current are suspended. 
 
     
     
       17. The method of  claim 16 , wherein the switching mode power supply transitions from the first mode to the second mode to the burst mode as the load decreases. 
     
     
       18. The method of  claim 16 , wherein the switching mode power supply further operates in an additional third mode during which the peak current limit varies as a function of the feedback signal and the switching frequency is constant. 
     
     
       19. The method of  claim 18 , wherein the switching mode power supply transitions from the first mode to the second mode to the third mode to the burst mode as the load decreases. 
     
     
       20. The method of  claim 18 , wherein the switching mode power supply transitions from the third mode to the second mode to the first mode to the burst mode as the load decreases. 
     
     
       21. A method for controlling a switching mode power supply, comprising:
 regulating an output voltage of the switching mode power supply by switching a transformer on and off at a switching frequency; 
 generating a feedback signal that varies in relation to a load coupled to the output voltage; 
 controlling the switching frequency based on the feedback signal; and 
 limiting a peak current through a primary winding of the transformer based on the feedback signal; and 
 
       wherein the switching mode power supply operates in three modes:
 a first mode during which the peak current limit varies as a function of the feedback signal and the switching frequency is constant; 
 a second mode during which the switching frequency varies as a function of the feedback signal and the peak current limit is constant; and 
 a burst mode during which the switching frequency and the peak current limit are suspended. 
 
     
     
       22. The method of  claim 21 , wherein the switching mode power supply transitions from the first mode to the second mode to the burst mode as the load decreases. 
     
     
       23. A multi-mode controller for a switching mode power supply that includes a transformer having a primary winding and a secondary winding to supply power to a load and a feedback circuit that generates a feedback signal that varies in relation to the load on the secondary winding, the multi-mode controller comprising:
 a switching circuit to be coupled to the primary winding to control current flow through the primary winding; 
 a frequency control circuit to control a switching frequency of the switching circuit based on the feedback signal; 
 a current limiting circuit to limit current flow through the primary winding by causing the switching circuit to suspend current flow through the primary winding when the current reaches a peak current limit that is set based on the feedback signal; 
 a burst mode control circuit that bypasses the frequency control circuit and the current limiting circuit to control operation of the switching circuit when the load on the secondary winding falls below a minimum load threshold; and 
 
       wherein multi-mode controller operates in three modes:
 a first mode during which the switching frequency varies as a function of the feedback signal and the peak current limit is constant; 
 a second mode during which both the switching frequency and the peak current limit vary as a function of the feedback signal; and 
 a burst mode during which operation of the frequency control circuit and the current limiting circuit is suspended. 
 
     
     
       24. The multi-mode controller of  claim 23 , wherein the multi-mode controller transitions from the first mode to the second mode to the burst mode as the load on the secondary winding decreases. 
     
     
       25. The multi-mode controller of  claim 23 , wherein the multi-mode controller further operates in an additional third mode during which the peak current limit varies as a function of the feedback signal and the switching frequency is constant. 
     
     
       26. The multi-mode controller of  claim 25 , wherein the multi-mode controller transitions from the first mode to the second mode to the third mode to the burst mode as the load on the secondary winding decreases. 
     
     
       27. The multi-mode controller of  claim 25 , wherein the multi-mode controller transitions from the third mode to the second mode to the first mode to the burst mode as the load on the secondary winding decreases. 
     
     
       28. The multi-mode controller of  claim 23 , wherein the frequency control circuit includes a voltage-controlled oscillator circuit that controls the switching frequency of the switching circuit as a function of a threshold voltage. 
     
     
       29. The multi-mode controller of  claim 28 , wherein the threshold voltage is determined by the feedback signal during at least one operational mode of the multi-mode controller, and wherein the threshold voltage held constant during at least one other operational mode of the multi-mode controller. 
     
     
       30. The multi-mode controller of  claim 29 , wherein the threshold voltage is held constant by a Zener diode circuit. 
     
     
       31. The multi-mode controller of  claim 29 , wherein the threshold voltage is held constant by a voltage source. 
     
     
       32. The multi-mode controller of  claim 23 , wherein the current limiting circuit includes a comparison circuit that controls the peak current limit as a function of a threshold voltage. 
     
     
       33. The multi-mode controller of  claim 32 , wherein the threshold voltage is determined based on the feedback signal during at least one operational mode of multi-mode controller, and wherein the threshold voltage held constant during at least one other operational mode of the multi-mode controller. 
     
     
       34. The multi-mode controller of  claim 33 , wherein the threshold voltage is held constant by a voltage source. 
     
     
       35. The multi-mode controller of  claim 23 , wherein the burst mode control circuit includes a comparison circuit that suspends current flow through the secondary winding based on the feedback signal. 
     
     
       36. A multi-mode controller for a switching mode power supply that includes a transformer having a primary winding and a secondary winding to supply power to a load and a feedback circuit that generates a feedback signal that varies in relation to the load on the secondary winding, the multi-mode controller comprising:
 a switching circuit to be coupled to the primary winding to control current flow through the primary winding; 
 a frequency control circuit to control a switching frequency of the switching circuit based on the feedback signal; 
 a current limiting circuit to limit current flow through the primary winding by causing the switching circuit to suspend current flow through the primary winding when the current reaches a peak current limit that is set based on the feedback signal; 
 a burst mode control circuit that bypasses the frequency control circuit and the current limiting circuit to control operation of the switching circuit when the load on the secondary winding falls below a minimum load threshold; and 
 
       wherein the multi-mode controller operates in three modes:
 a first mode during which the peak current limit varies as a function of the feedback signal and the switching frequency is constant; 
 a second mode during which the switching frequency varies as a function of the feedback signal and the peak current limit is constant; and 
 a burst mode during which operation of the frequency control circuit and the current limiting circuit is suspended. 
 
     
     
       37. The multi-mode controller of  claim 36 , wherein the multi-mode controller transitions from the first mode to the second mode to the burst mode as the load on the secondary winding decreases.